The invention relates to a method for measuring the wall thickness of a tubular object. The method is suitable, for example, for contactless measurements of the wall thickness of a steel tube by using beams of high penetrating power.
Usually, the wall thickness of pipelines is measured using projection radiography or projection radioscopy. For this purpose, a pipeline with or without insulation is transradiated by means of an X-ray tube for a gamma radiator. A film or other planar detector which is fixed at right angles to the beam direction is used to take a projection image of the pipeline which is located between radiation source and pipeline. The shadow image of the tube wall on the film on another detector is classically measured using a length measuring instrument (for example a ruler or calliper) after visual assessment. The wall thickness results after correction of the measured values using the known magnification ratio of the wall image. A problem in this case is that the visual determination of the wall limits to be measured is possible only imprecisely, because of radiated overexposure and unsharpness, and varies very strongly from evaluator to evaluator.
Various systems have been patented and described in order to circumvent this problem of subjective evaluation by humans, and these permit computer-aided evaluation and automate the measuring sequence in order to avoid measuring errors.
When a beam, for example an X-ray beam, traverses an object, the intensity of the beam decreases because of absorption in the object. The intensity of the beam can be considered as the number of photons, and also as the counter reading of radiation detectors. The physical relationships are reproduced by the following equation for the primary radiation, which forms the image:
I=I0*EXP(xe2x88x92xcexcx)xe2x80x83xe2x80x83(1)
I being the intensity of the beam after traversing the object, I0 the intensity of the beam before penetrating the object, EXP the base of natural logarithms, xcexc an absorption coefficient, which depends on the material and energy, and x the length of the traversal path of the beam in the object.
In addition to absorption, the beams which traverse an object also undergo scattering. The influence of this is taken into account by the so-called build-up factor B:
I=I0*B*EXP(xe2x88x92xcexcx)xe2x80x83xe2x80x83(2)
B being greater than or equal to one. The build-up factor B is variable. It depends on the distance between the object and the beam detector, on the geometry and on the material of the object.
FIG. 1 shows a known method for measuring the wall thickness of a steel tube by using beams, and is disclosed in Japanese Laid-Open Patent Application No. 114263. FIG. 1 shows a tubular object (related by the number 1) which will be referred to below as xe2x80x9ctubexe2x80x9d, for short, and whose wall thickness is to be measured. It is assumed that the tube is of cylindrical shape and therefore that its outer surface has a radius R1 and its inner surface a radius R2. An X-ray beam 2, which is directed perpendicular to the tube serves to scan the tube. The X-axis reproduces the traversal path x of the beam in the tube wall. The y-axis runs at a right angle to the tube axis, and thus also at a right angle to the tube wall. The I-axis reproduces the determined intensity I of the beam after traversal of the tube. A typical profile of an intensity curve (I as a function of y) is demonstrated in FIG. 1. The sites of the points of inflection S1 and S2 or S3 and S4 are determined on a measured curve P, the distance between the points of inflection S1 and S2 or S3 and S4 corresponds to the wall thickness of the tube. The point of inflection S1 or S4 is at the site where the beam runs tangential to the outer surface of the tube, and the attenuation of the beams is still minimal. The point of inflection S2 or S3 is at the site where the beam runs tangential to the inner surface of the tube and the attenuation of the beams is maximal.
A further known method for measuring the wall thickness of a steel tube by using beams is disclosed in German Laid-Open Patent Application No. 3123685 A1. In this method, the intensity curve in FIG. 1 is decomposed in at least three areas, and each of these sections is approximated by an equation. By solving these approximate equations, the y-co-ordinates of the points of inflection are determined, and the wall thickness is subsequently determined by subtraction.
This known method yields an inadequate accuracy in determining tube wall thickness, since it is virtually impossible exactly to find the points of inflection with an actual measurement because of unsharp imaging of the beam and of scattering of the beam in the material, particularly when the tube is filled with a liquid.
A very narrow beam is required in order to obtain a sharp image of the tube. Reducing the thickness of the beam leads to a reduction in the beam intensity which reaches the beam detector per unit of time. In order to reach the desired signal level, it is necessary to lengthen the measuring time up to several minutes per measuring point in accordance with the reduction in the thickness of the beam. The result of this is a measuring time of a few hours for each determination of the tube wall thickness.
The scattering of the beam continues to occur even with the reduction in the thickness of the beam. Reliable prediction of the extent of the scattering, the build-up factor B, and the correction resulting therefrom for the unsharpness owing to scattering is virtually impossible, particularly when the tube is filled with a liquid which causes additional scattering.
The use of radiation shields, so-called collimators reduces the scattering. A collimator is designed such that a planar slit which is formed by two rectangular solid radiation screens permits the beam path to pass in a geometrically defined fashion. The use of a collimator describes the direction of the measurement of the wall thickness, that is to say the measurement of the wall thickness can be performed only in alignment with the collimator. For this reason, the collimator is aligned perpendicular to the tube. The measurement of the wall thickness in a tube bend wrapped with insulation is virtually impossible with the collimator, because it is not possible to ensure the collimator is aligned exactly perpendicular to the covered tube surface.
As has been shown above, the known method is not feasible for actual application in measuring wall thickness; this holds, in particular, for inspecting pipelines in chemical plants and in refineries.
European patent application 0 009 292 A discloses a method of determining the thickness of a tubular object. The tubular object is tangentially irradiated from its outer side. From maxima and minima of an irradiation pattern obtained in a plane which is perpendicular to the axis of the tubular object the wall thickness is calculated.
It is an object of the invention to provide a method for measuring the wall thickness of at least partially tubular objects that overcomes the problems of the prior art.
The method of the present invention is suitable for use in inspecting objects that are at least partially tubular such as the pipelines in chemical plants and in refineries. It is suitable for measuring wall thickness under real operating conditions, that is to say when the partially tubular object is empty or filled e.g. when a fluid is flowing through the tube, when the surface temperature of the tube is in the range from xe2x88x92120xc2x0 C. to +400xc2x0 C., when the tube is covered because of the insulation, and when the pipeline runs in various sweeping tube bends.
The method of the invention comprises the following steps:
a tubular part of an object is transradiated by radiation emitted by a radiation source,
the radiation image of said tubular part is recorded with the aid of a radiation detector,
the radiation image is converted into a digital image,
an attenuation profile is taken from the digital image on a straight line, the attenuation profile comprising a tangential image of the wall of the tubular part of the object and a section outside the tubular object and a section inside the tubular object,
the attenuation profile is reflected in the direction of the centre of said object to obtain a measured reflected attenuation profile,
a relative density distribution of the object is reconstructed by means of a transmission tomography-projection reconstruction method in which the measured reflected attenuation profile is used for all annular positions,
a density profile is selected from this density distribution by scanning on a straight line through the midpoint of the object, this density profile reproducing the wall of at least the tubular part of the object with its bordering surroundings, the wall being represented by an emphasised section because of the difference in density relative to its bordering surroundings,
positions of outer and inner surface of said wall are determined from said density profile, and
the wall thickness is determined from the spacing between the positions of the outer and inner surfaces of the wall.
The radiation detector may be an X-ray film, a line camera, a storage phosphor, an X-ray image intensifier or a detection system based on semiconductors or the like.
The positions of the outer and inner surfaces of the wall are determined in a region between 25% and 75% of the height of the density profile in the emphasised section. Preferably 50% of the height of the density profile in the emphasised section is taken.
The radiation source may be an X-ray tube or a gamma radiator which is arranged opposite the radiation detector, the object being located between the radiation source and radiation detector.
The measured wall thickness is then preferably corrected by a magnification factor resulting from the recording geometry.
In another embodiment the radiation source and the radiation detector are arranged in such a way that a parallel beam projection profile is produced.
In order to determine the wall thickness from radiation projection profiles of at least partially tubular objects with a constant wall thickness, the reflected profile may be replaced by a profile over the entire cross section of the object.
The reflection of the measured profile can be dispensed with and that the projection reconstruction can be carried out over an angle of 90 degrees.
A series of values of constant intensity may be added to the attenuation profile at the site of the reflection point, this constant intensity being determined from measured intensity values which are the nearest to the reflection point being fixed in the centre of this series of constant intensity values.
The wall thickness may be determined from the spacing between the extreme values of the derivative of the reconstructed density profile.
The reconstructed density profile may be smoothed before or after the derivation, and the wall thickness may then be determined from the spacing between the extreme values of the derivative.
It is also possible to take a plurality of parallel, neighbouring attenuation profiles along the wall of the tubular object from the digital image, and to calculate a new attenuation profile by combining these attenuation profiles, and then to use this attenuation profile to calculate the relative density distribution as set out higher.
The number of the points of the measured profile or of the reconstructed density distribution in case of the non-constant as well as in case of constant wall thickness may be increased by interpolation, so that the wall thickness is determined with a higher resolution.